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Journal of Plant Pathology (2010), 92 (1, Supplement), S1.79-S1.85 Edizioni ETS Pisa, 2010 S1.79

MINIREVIEW

BACTERIAL DISEASES OF WALNUT AND HAZELNUTAND GENETIC RESOURCES

D. Frutos

Instituto Murciano de Investigacin y Desarrollo Agrario y Alimentario (IMIDA).30150, La Alberca, Murcia, Spain

SUMMARY

In 2008, a global production rounding 1,724 and1,053 million metric tones respectively for walnut andhazelnut has been estimated. Bacterial diseases arethreatening these nut crops all over the World. Xhan-thomonas arboricola pv. juglandis (Xaj) is the agent of

walnut bacteriosis, and has been associated also tobrown apical necrosis. Several walnut genotypes haveshown a range of tolerance to this disease in diversecountries. Deep bark canker (Brenneria rubifaciens) andShallow bark canker (Brenneria nigrifluens) are the oth-er bacterial diseases affecting walnut. Being polyphenolsinvolved in tolerance to bacterial diseases, it has beenshown that the genejrPPO1 is the sole polyphenol oxi-dase PPO gene in walnut able to encode a jrPPO en-zyme that is expressed in the leaves, hulls and flowers ofwalnut trees. It can also happen that some modificationsin the anatomical traits of the epidermis of walnut prog-enies, such as hair density or wax thickness could act asshields against Xaj infection. These changes could ex-plain segregation of tolerance in walnut progenies. Arange of tolerance to hazelnut blight, induced by Xan-tomonas campestris pv. corylina, has been observed inFrance in diverse nursery plants. Pseudomonas syringaepv. avellanae, observed first in northern Greece, is af-fecting hazelnuts also in Italy, where this disease it elicitsis known as moria (death). No resistance or toleranceagainst moria has been found.

INTRODUCTION

Walnut (Juglans regia L.) production in the world hasbeen estimated around 1,724 million metric tonnes(mMT): Asia (956) [China (503), Turkey and Iran (170),India (37)], America (395) [USA (290), Mexico (69),Chile (23), Argentina (10)]; Europe (332) [France (36),Romania (32), Italy and Germany (17 each), Greece(15), Spain (10)]. Likewise, hazelnut (Corylus avellana

L.) world production has been estimated around 1,052mMT: Asia (873) [Turkey (800), China (15)], Europe(145) [Italy (111), Spain (18), France (5), Greece (2,5)],and USA (33) (Faostat, 2008). The main bacterial dis-eases that threaten both crops and possibilities to controlthem by using genetic resources are discussed below.

BACTERIAL DISEASES OF WALNUT

Walnut blight. Xanthomonas arboricola pv. juglandis(Xaj) is the causal agent of walnut blight, the most im-portant bacterial disease of Juglans regia and otherJuglans species, which affects a high percentage of pistil-late flowers and fruits, but does not kill bearing trees.This bacterium has been also isolated from tissues affect-ed by bacterial apical necrosis (Belisario et al., 2002).Blight has been recorded from Europe (Austria, Bulgar-ia, Denmark, France, Germany, Greece, Italy, Moldova,

Netherlands, Poland, Portugal, Romania, Russia, south-ern Russia, Slovenia, Spain, Switzerland, UK, Ukraineand former Yugoslavia) Asia, [Azerbaijan, China,(Hebei, Henan, Jiangsu, Shaanxi, Shandong), Georgia,India, (Himachal Pradesh, Uttar Pradesh), Iran (Qazvin,Hamadan, Tehran, Markazi, Aradabil, Guilan, Mazan-daran and Golestan provinces (Golmohammadi et al.,2002)], Iraq, Israel, Lebanon and Uzbekistan], Africa(South Africa and Zimbabwe), North America (Canadaand Mexico), USA (Alabama, Arkansas, California, Con-necticut, Delaware, Georgia, Indiana, Kansas, Louisiana,Maine, Maryland, Michigan, Minnesota, Missouri, NewJersey, New York, Ohio, Oregon, Pennsylvania, Texas,

Virginia and Washington), Caribees (Bermuda), SouthAmerica (Argentina, Chile and Uruguay), and Oceania[Australia (New South Wales, Queensland, South Aus-tralia, Tasmania, Victoria, Western Australia) and NewZealand] (CABI-EPPO, 2001).

J. ailantifolia, J. ailantifolia var. cordiformis, J. californi-ca, J. cinerea, J. hindsiiandJ. nigra have been also record-ed as Xaj hosts (Miller and Bollen, 1946; Bradbury,1967). The disease was reproduced on tender shoots andleaves ofJ.nigra, J. californica, J. hindsii, J. cinerea, J. ailan-tifolia, J. ailantifolia var. cordiformis, and Paradox (J.hindsiix J. regia) and Royal (J. nigra x J. regia) hybrids

Corresponding author: D. FrutosFax: +34.968366792E-mail: [email protected]

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(Smith, 1914; Smith et al., 1912). J. mandshurica, J.cinerea, J. regia, J. nigra, J. cordiformis.J. x bixbyiRedh. (J.cinerea x J. ailantifolia) and J. x quadrangulata (Carr.)Redh. (J. cinerea xJ. regia ) hybrids have been also listedasXajhosts in Lithuania (Burokiene, 2009).

Symptoms of the disease consist of dark brown toblack spots on new leaves, stems and nuts. Many nuts

fall prematurely; others reach full size, but their kernelbecome blackened, dried and wrinkled. Rain is the mostimportant dispersal agent (Miller and Bollen, 1946; Pic-cirillo, 2003) but insects and mites may also carry thebacteria (Smith et al., 1912). The presence of aphids(Chromaphis juglandicola) did not increase the percent-age of affected fruits, but it favored a higher incidenceofXaj on the leaves (Arquero et al., 2006). Pollen frominfected catkins can contain Xaj. Miller and Bollen(1946) did not consider pollen transmission to be im-portant in Oregon (USA) although several experimentssuggest that such transmission occurs (Ercolani, 1962,

cited by Bradbury, 1967). Main infection avenue isthrough stomata or wounds. The bacterium overwintersin diseased buds and twigs. Xajseverity increases withearliness of flowering (Alet, 2004; Piccirillo and Petric-cione, 2006).

It is possible to obtain plants tolerant to Xaj whentwo susceptible genitors are crossed (Rovira et al.,2007). On the other hand, the evaluation in the open inan ex situ collection showed a segregation of Xajtoler-ance in J. regia progenies (Frutos et al., 2008, 2009,2010; Ruiz-Garca et al., 2010). These segregationsfound in natural conditions suggest that anatomicchanges in the epidermis of young twigs, leaves andflowers could act as barriers againstXajinfection.

Table 1 summarizes the information from differentcountries on tolerance/resistance/ low susceptibility toXajinJ. regia.

Polyphenols and Xaj. It is known that polyphenoloxidase (PPO) catalyzes the oxidation of polyphenols toquinones, which are very reactive against disease agents.The PPO-encoding gene jrPPO1 from a walnut pistil-late flower cDNA library has been cloned and shown tobe the sole gene of walnut encoding a jrPPO enzymeexpressed in the leaves hulls and flowers of the host (Es-

cobar et al., 2008). When a bacterial infection occurs, anecrosis develops in nuts or leaves resulting in their ab-scission. In this mechanism of defense phenolic com-pounds as juglone and their glucosides are involved,which are induced even when a very low Xajcontami-nant occurs (Garcin and Duchesne, 2001). Matias et al.(2009) found that cv. Franquette had lower contents ofgallic acid, vanilic acid and juglone than cv. Hartley, butblight symptoms were significantly stronger in the latter.On the other hand, no correlation between damageselicited by artificial infections and percentage of con-taminated fruits in the open field was found for cvs.

Fernet, Erjavec, Cisco, Zdole, Seiferdorfski and Sampi-on, although, after infection, an accumulation of differ-ent phenolic compounds was detected in both naturaland artificial inoculation. There is some evidence of aprotective function of some phenolic compounds as ju-glone, ellagic acid and hyperin (Solar et al., 2007, 2009).Except for the very sensitive cultivars,Xajdamages are

not extensive in oldJ. regiawood. Taking into accountthis fact, it would be interesting to evaluate if such oldwood contains higher levels of phenolics than leaves oryoung twigs or nuts. This information may lead to a bet-ter understanding of the role of polyphenols inXajcon-trol.

Deep bark canker. This disease (DBC) induced byBrenneria rubifaciens is characterized by cracks in scaf-fold branches and trunks, where infection is first estab-lished. The disease advances slowly upward, affectingmore and more branches which are weakened, while the

crop decreases progressively. The symptoms do not ex-tend into the rootstock. Cankers ooze a characteristicreddish to dark-brown substance from late springthrough early fall. Hartley has been reported as themost susceptible cultivar to DBC. High temperaturesfavour the development of DBC, which makes this dis-ease to prevail in the Central Valley rather than in thecoastal areas of California (Anonymous, 2007a). Bren-neria rubifaciens has been also isolated from cv. Hartleyin Badajoz (Spain) (Gonzalez, 2002). A specific test forthe early detection of DBC has been developed in mu-tants deficient in rubifacine (pig-), the pigment oozedfrom cankers. This test can be useful to investigate B.rubrifaciens ecology under both nursery and orchardconditions (McClean et al., 2008).

Shallow bark canker. The agent of shallow barkcanker (SBC) is Brenneria nigrifluens, whose symptomsconsist of brownish to black round spots which appearon trunk or on lower scaffold limbs. The bark at themargins of these spots presents a water-soaked appear-ance with the centre showing a drop of black ooze thatdries with time. These superficial cankers can be exten-sive, but they seem to cause little damage to the tree. Al-though SBC affects many commercial walnut cultivars,

it is not considered a major disease in California. As itoccurs with DBC, SBC can be severe in stressed trees(Anonymous, 2007b). SBC has been observed as muchin nursery as in orchards in cvs Fernor, Chandler,Mayette, and Hartley, although Lara and Franquettecould also be affected (Mnard et al., 2004). It has beenreported that SBC induces abortion and fall of fruits inan ex situ collection located at Pignataro Maggiore nearCaserta (Italy), where affected trees can die in few yearsafter infection (Piccirillo, 2003). Symptomatic treeswere observed also in Veneto, Piedmont and Campania,Italy. In these cases, cankers were deeper and more se-

S1.80 Diseases of walnut and hazelnut and genetic resources Journal of Plant Pathology (2010), 92 (1, Supplement), S1.79-S1.85

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vere than those reported from USA, resembling thosecaused by B. rubrifaciens (Loreti et al., 2006). B. nugri-fluent has been also observed on walnuts in Iran(Yousefikopaei et al., 2007). Identification tests for earlySBC identification have been developed (Loreti et al.,2008; Roshangar and Harighi, 2009).

BACTERIAL DISEASES OF HAZELNUT

Blight. In the 1970sXantomonas campestris pv. coryli-na (Xcc) (=X. corylina) was identified as the causal agentof hazelnut (Corylus avellana) blight in south-westernFrench orchards established with hazelnuts importedfrom Oregon, where the disease is endemic (Luisetti et

al., 1975). Later, the presence of Xcc was confirmed on

Journal of Plant Pathology (2010), 92 (1, Supplement), S1.79-S1.85 D. Frutos S1.81

Table 1.Juglans regia accessions resistant/tolerant toXajfound in native walnut population.

Country Locality Selections or cultivars Resistance/ tolerance References

Taigu, Shanxi Jinboxiang 3 Xajresistance in the open Tian et al., 2008

Kunming,Yunnan

Yunxin Gaoyuan, out ofJ.sigillataxJ.regia Yulin A7,cross made in 1977

Resistant toXajin theopen

Xi et al., 2007

Kunming,Yunnan

Yunxin 90303derivedfromJ. sigillatacv. SantaixJ. regiacv. Xinzao No. 13

Diseases resistant Zhao et al., 2007

Taian,Shandong Luguo 4, open pollinatednative genotype Xajresistant in the open Zhang etal., 2009

China

Ghengxiancounty, Gansu

Longnan 15 Xajresistant in the open Guo et al., 2009

Greece Amigo, Ashley, Serr,Eliana, Grand Jean,EK-1, EH-28, EH-29

The lowerXajsusceptiblein the trial conditions

Tsiantos et al., 2007

Italy PignataroMaggiore, areaof Caserta

Ex situcollection AllXajsusceptible Piccirillo, 2003Piccirillo andPetriccione, 2006

Oltenia 550 genotypes outof native population

Several genotypesareXajtolerant

Botu et al., 2001

Valcea Valcor, Valmit and Valrex,out of native population

Texting in orchard forXajand other traits

Botu et al., 2007

Romania

Valcea Portval(syn. VL 26 B),seedling rootstock

Good resistance todiseases,Xajnot specified

Achim et al.,2007

Galicia >550 natural genotypetolerant in the open wereevaluated for Xaj,susceptibulity

MBLu-20, MBLu-21and MBC-45, did showlowXajsusceptibility inlab.

Aleta et al., 2001

Cantabria Native seedlings aged over15-years-old

28 were selected asXajfree symptoms. In lab all

were susceptible

Arrieta and Diaz,2007; Lopez et al.,2007

Spain

Murcia Ex situcollection Segregation toXajtolerance were found in anex situcollection

Frutos et al., 2008,2009, 2010; Ruiz etal.,2010

Slovenia 24 genotypes preselected Most promising (Xaj

resistant) are late leafinggenotypes Z-62, Krncand Z-60

Solar and Stampar,

2006

19 genotypes All genotypesXajresistant Akca and Ozongun,2004

Turkey

Sen 2, C440, 77 H-1, Kaplan86, Tokat 1, Kaman 1and Bilecik

All selectionsXajlowsusceptible

zaktan et al., 2007

Nbs. 10/88, 40/92, 30/93,28/94, 9/96, selected fromnative population

Highly resistant to diseases Mitrovic, 2003Yugoslavia

Nbs. 10/88, 40/92, 30/93,28/94, 9/96

All resistant toXaj Mitrovic et al., 2007

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hazelnuts in several locations of Victoria (Australia),where blight is widespread, suggesting that it had beenpresent for many years in the area before its discovery(Wimalajeewa and Washington, 1980). In 1985, Xcc-in-duced symptoms, i.e. death of buds and new shoots,cankers on branches and trunks, leaf spots, dark brownspots on nuts and bacterial exudates on necrotic lesions,

were observed on 4-year-old trees of cv. Barcelona in Car-illanca (Chile). Old hazelnut trees were less affected thanthe young ones (Guerrero and Lobos, 1987).

Blight has been also reported from Turkey (Black Seaarea), UK, Italy (Civilleri et al., 2007), former Yu-goslavia, Russia (Bradbury, 1987; Anonymous, 1996),and Iran [Guilan province (Kazempour et al., 2006; Aliet al., 2006)]. In countries like Turkey, Italy and USA,

which account for the bulk of hazelnut production (Vig-giani, 1994), blight is regarded as the most importantdisease of this crop. The main problem of the Italianand Spanish hazelnut industry is the selection of vari-

eties resistant to Xcc attacks (Avanzato et al., 2009).Other hosts ofXccare Corylus maxima, C. rostrata (= C.californica) and C. colurna, which proved susceptible

when artificially inoculated (Bradbury, 1987).Disease symptoms consist of buds and new shoots

death, leaf spots, dark-brown spots on nuts, bacterialexudates on necrotic lesions, cankers on branches andtrunks which may girdle these organs, including trunksof young trees, thus killing the parts of the tree abovethe lesions.Xccoverwinters in cankers on branches andbuds, and spreads to other parts of the tree in wet

weather. Limited spread from tree to tree takes place bywater splash. Man is an important agent of bacterial dis-semination, especially by pruning young trees with con-taminated cutting tools and is also responsible for theintroduction of the disease into new areas with infectedplanting material (Bradbury, 1987; Anonymous, 1996).The potential for natural spread can be considered asrelatively low. However, seeds of fruits picked from in-fected trees can yield infected seedlings (Bogatzevska,2007). The most important factor for reducing mortalityfrom Xcc in newly established hazelnut plantings is anadequate irrigation in the first 2-3 years after planting(Moore et al., 1974).

As to reaction of hazelnut cultivars toXccattacks, pre-

liminary observations in nursery showed that cvs. Fertilede Coutard is highly susceptible (Luisetti et al., 1975;Prunier et al., 1976), Ronde de Piemont shows an inter-mediate level of susceptibility, Imperatrice Eugenie isslightly susceptible and Negret, Merveille de Bollwillerand Segorbe are almost resistant (Prunier et al., 1976).

Bark canker and decline Pseudomonas syringae pv.avellanae, (Psa) [= P. avellanae (Scortichini, 2002)] is thecausal agent of hazelnut bacterial canker and decline inGreece and Italy (Wang et al., 2007). This disease wasfirst observed in northern Greece in 1976 (Psallidas and

Panagopoulos, 1979) then in Italy, where is known underthe name of moria(decline and death) and did spreadrapidly, particularly in the province of Viterbo (Scorti-chini and Tropiano, 1994). Environmental factors playan important role in disease spread. For example, in aplantation with prevailing south-west winds, the diseasemoved progressively year after year to the south and

west (Scortichini and Martins, 2000). Frost and hail in-juries may also favor disease spread (Scortichini andMartins, 2000). In the course of surveys in forests of cen-tral Italy carried out in 1996-98 to verify the possiblepresence of bacterial canker in wild hazelnut trees, wilt-ed twigs were found several times, especially in summer.Moreover, wild hazelnut trees growing near commercialorchards appeared completely wilted (Scortichini et al.,2000). Psa has been found in soils of orchards affectedby moria. Infections were favoured by high soil acidi-ty, poor cultivation practices as planting very deep, useof acid fertilizers such as ammonium sulfate, high levels

of aluminum in soils, and high levels of iron and man-ganese in the roots (Scortichini et al., 2001). Surveys car-ried out in the other main areas of hazelnut cultivation inItaly determined the presence of bacteria similar toPseudomonas syringae pv. syringae van Hall (Pss), but se-vere cases of bacterial decline were not observed. Psa at-tacks, always of high virulence, are restricted to Corylusavellanae (Scortichini, 2002; Scortichini et al., 2002), asexemplified by the killing of more than 1800 adult hazel-nut trees in a single orchard within 5 years (Scortichiniand Tropiano, 1994). On a wider scale, the death ofmore than 40,000 trees on a surface of 20,000 ha planted

with hazelnut in Latium (central Italy) has been attrib-uted to moria, which continues to damage trees on ap-proximately 1000 ha (Scortichini, 2002).

The study of thirty-eight bacterial strains isolatedfrom cv. Tonda Gentile delle Langhe showing twigdieback in Piedmont and Sardinia (Italy), justified thecreation of a new pathovar denoted P. syringae pv. coryli(Psc) (Scortichini et al., 2005). Field surveys for evaluat-ing the sanitary status of hazelnuts orchards in theprovinces of Catania and Messina (Sicily), disclosed thefrequent occurrence of twig and branch dieback con-nected with the presence of Psc(Cirvilleri et al., 2007).

DISCUSSION

The notion that Xaj can infect a very wide range ofJuglans species and their interspecific hybrids (Smith etal., 1912; Smith, 1914; Miller and Bollen, 1946; Brad-bury, 1967; Burokiene, 2009), prompted the search forJ. regia genotypes showing high tolerance to this bacteri-um. Crossing susceptible-to-Xajwalnut materials yield-ed segregation of genotypes (Frutos et al., 2009, 2010;Ruiz et al., 2010), which suggested that the presence ofanatomical differences in epidermal tissue, such as hairs


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density and/or wax layer thickness, acted as a shieldagainstXajentry into walnut tissues.

DBC disease is less important than Xaj-induced api-cal necrosis, althoughB. rubifaciens can produce impor-tant damages to cv. Hartley, the most sensitive cultivar,

which is becoming progressively less popular.SBC appears more virulent in some European coun-

tries (Loreti et al., 2006; Piccirillo, 2003) than in Cali-fornia, where it is a disease of minor importance(Anonymous, 2007b).

Xcccould be to hazelnut whatXajis to walnut. Somehazelnut cultivars have a low susceptibility to Xcc(Prunier et al., 1976) thus could be used as parents fornew genotypes highly tolerant to it. But for bark cankerand decline caused by Psa, no resistant/tolerant culti-vars have been found in C. avellana.

More tolerant genotypes to bacterial diseases seem toexist in walnut than in hazelnut. This fact may be con-nected with the propagation procedure. Whereas seeds

have been traditionally used for propagating walnuts,vegetative propagation represents the norm in hazelnut.It ensues that more gene recombination and segregationoccur in walnut.

Advances in knowledge of phenolic compounds inwalnut led to think that the higher is phenolic content,the higher is tolerance to Xaj. However, the oppositehas been also pointed out, i.e., the more phenolics, thelesser tolerance (Matias et al., 2009). Laboratory testingfor phenolic content were unable to find any walnutgenotype resistant toXaj, whereas a number of tolerantgenotypes free of blight in the open field have been ob-served in several countries. This is likely dependent onanatomical changes in the epidermis commented above,i.e hairs and wax acting as shields againstXaj. However,

when the bacterium breaks this barrier, its progress can-not be stopped, but can only be slowed down in culti-vars with higher phenolic content.

CONCLUSIONS

The most important bacterial diseases in walnut andhazelnut seem to be due to Xaj and Xcc, respectively.Both pathogens have a worldwide distribution.

Hazelnut blight and decline disease (Psc) has pro-duced important losses to hazelnut orchards and forestsin Italy.

DBC (Brenneria rubifaciens) and SBC (B. nigrifluens)seem to be of lesser importance, although B. nigrifluenshas produced more damages in some European coun-tries than in California.

Whereas laboratory tests for phenolic content havenot disclosed any correlation with resistance to Xaj,many blight-free walnut genotypes symptoms have beenobserved in the open field, possibly consequent toanatomical changes in epidermis of walnut progenies.

Concerning hazelnut blight disease, some cultivarsseem to be more tolerant than others, but no real resist-ance has been found.

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